The present invention relates to an improved method for the suspension polymerization of vinyl chloride monomer or, more particularly, to a method for the suspension polymerization of vinyl chloride monomer in which the time taken for a run of the polymerization can be remarkably decreased without affecting the quality of the resultant polyvinyl chloride resin product in respect of the number of fish eyes, velocity of plasticizer absorption and so on.
As is well known, suspension polymerization vinyl chloride is performed in most cases in a batch-wise process, in which, conventionally, a polymerization reactor is charged with water for the aqueous medium, dispersing or suspending agent, polymerization initiator and vinyl chloride monomer together with or without other optional additives and the thus prepared polymerization mixture is heated under agitation at a prescribed polymerization temperaturekept constant throughout the run to effect the polymerization of the monomer.
In compliance with the technological desire in recent years to increase the productivity in the above described process of the suspension polymerization of vinyl chloride, in particular, in respect of the utilization efficiency of, among others, the relatively expensive polymerization reactor, various attempts and proposals have been hitherto made for reducing the overall length of time taken for the completion of a run of the polymerization procedure composed not only of the time for the polymerization reaction per se but also of the times for the ex-reaction procedures including the time for the maintenance and cleaning of the polymerization reactor after the preceding run, the time taken for the introduction of the above mentioned constituents of the polymerization mixture into the reactor, the time for the evacuation of the reactor before the start of the polymerization reaction, the time for the discharge of the polymerizate slurry out of the polymerization reactor and so on.
Needless to say, reduction of the length of time taken for the polymerization reaction per se is of the major importance from the standpoint of increasing the overall productivity of the run so that attempts have also been made in this direction, for example, by increasing the polymerization temperature or by increasing the amount of the polymerization initiator introduced into the polymerization mixture. Such a conventional method, however, is practically not feasible because of the disadvantages that the polyvinyl chloride resin product produced by such a modified method may sometimes contain a greatly increased number of fish eyes and the velocity of plasticizer absorption into the resin product may be greatly decreased. The increase of the number of fish eyes, if alone, can be compensated by the addition of a small amount of a polymerization inhibitor to the polymerization mixture at the early stage of the polymerization reaction. This method of adding a polymerization inhibitor, however, more or less retards the polymerization reaction and also has a problem that the polyvinyl chloride resin product obtained thereby may sometimes be colored.
Accordingly, it is a very important technical problem requiring an urgent solution to develop an improved method for the suspension polymerization of vinyl chloride monomer in which the length of time taken for a run of the polymerization reaction can be reduced remarkably or significantly without affecting the quality of the resin product produced thereby.